Superconductivity is a phenomenon of zero electrical resistance and expulsion of magnetic fields occurring in certain materials when cooled below a characteristic critical temperature. Superconductors are used to build Josephson junctions which are the building blocks of superconducting digital electronics for superconductive computers, Josephson based sensors, such as superconducting quantum interference devices (SQUIDs), and quantum computing devices. Magnetic memories are also used in superconductive computers, where the environment is very cold and the traditional transistors (or pn-junctions) are not practical, because of the inefficiency of their intrinsic energy usage compared to Josephson Junction circuits or a magnetic memory circuit.
Magnetoresistive or Magnetic random-access memory (MRAM) is a non-volatile random-access memory (RAM) that stores data in magnetic storage elements. The magnetic storage elements are typically formed from two ferromagnetic plates separated by a thin insulating layer. Each of the ferromagnetic plates can hold a magnetization, with one of the two plates being a permanent magnet with a specific polarity, but the magnetization of other plate can be changed to match that of an external field to store data. This configuration is known as a spin valve and is a simplified structure for an MRAM cell. A magnetic RAM device can then be formed from a grid of such spin valve cells.
A spin valve is a device that includes two or more conducting magnetic materials, the electrical resistance of which can change depending on the relative alignment of the magnetization in the layers. The electrical resistance change is a result of the Giant Magnetoresistive effect, which is a quantum mechanical magnetoresistance effect in thin-film structures formed from alternating ferromagnetic and non-magnetic conductive layers. The magnetic layers of a spin valve device align directionally, for example, up or down, depending on an external magnetic field applied to the device. In a simple case, a spin valve device consists of a non-magnetic material sandwiched between two ferromagnets, one of which is fixed (pinned) by an antiferromagnet which acts to raise its magnetic coercivity and behaves as a “hard” layer, while the other is free (unpinned) and behaves as a “soft” layer. Due to the difference in coercivity, the soft layer changes polarity at lower applied magnetic field strength than the hard one. Upon application of a magnetic field of appropriate strength, the soft layer switches polarity, producing two distinct states consisting of a parallel, low-resistance state, and an antiparallel, high-resistance state.
A spin transfer torque (STT) is an effect that modifies the orientation of a magnetic layer in a spin valve device and can be changed using a spin-polarized current. STT uses spin-aligned (“polarized”) electrons to directly torque a nearby layer. Specifically, if the electrons flowing into a layer have to change their spin, this will develop a torque that will be transferred to the nearby layer. This lowers the amount of current needed to write to the cells of an MRAM, making it similar to a read process of the MRAM.
Spin Hall Effect (SHE) is a transport phenomenon for the appearance of spin accumulation on the lateral surfaces of a sample carrying electric current. The opposing surface boundaries have spins of opposite sign. SHE can be used to electrically manipulate electron spins. For example, in combination with the electric stirring effect, SHE leads to spin polarization in a localized conducting region.
One of the substantial problems with current magnetic memory architectures is that they cannot perform “bit select,” without using traditional transistors. Typically, one or more three terminal device such as a transistor is used at each memory cell location to individually select a bit or word in a memory array for read and writing operations. Another problem with magnetic memory array architectures is the relative high density of such array resulting in smaller memory capacity and/or larger packaging size. Moreover, most of the existing magnetic memory architectures are unscalable, electrically incompatible or energy inefficient.